Riding GF99 instead of mandatory/safety stops

[tbone1004]

Okay wrong word choices. Got it. Just trying to make a difficult subject a little easier to understand. I will refrain from commenting further.

Remember that this board is visited by thousands of unique people every day and that it is also fully searchable by Google which makes it the only large and active source of this information on the internet right now. Facebook is horrifically unreliable for learning, especially if it's more than a few hours old, and most of the other forums have died. When you make a factual error in a post, from a community learning standpoint it is important to point out errors. It's not a personal attack on your or your knowledge, hell everyone makes mistakes but it's important to realize where a lot of the feedback is coming from.

Now, the above paragraph was written before I actually read the post in its entirety so here is what I would say in response to at least the first bit, I assume the rest has been addressed.

It should be noted that the skin is acknowledged as a fast tissue, so it is important that while we don't call out all 16 compartments as being analogous to specific tissues, we do know that skin is one of the fast tissues so definitely important to not put it in the slow grouping. When I make those comparisons I usually use skin as a fast tissue and bone as a slow tissue but don't assign them to any specific compartments or use anything in the middle.


@OctopusLover the best you will get from the decompression doctors is that they are personally diving 50/70 and that they've found that a gf low of roughly 80% *plus or minus a few to hit convenient round numbers, i.e. 50/70 or 60/80, etc* of gf-hi seems to exhibit the least amount of decompression stress markers in the body. This is still pre-COVID values, but that's as close to a recommendation as you will find and is the most recent state of the art as of today. This was in direct response to a study by Spisni et al sponsored by UTD to promote deep stops and actually got them to change their ascent profiles which was truly shocking.

 

[kaylee_ann]

Why do I keep seeing 40/70 as a commonly used profile for tech diving then? Are you saying modern thinking is 80/80 or something flat like that?

From what I remember, a GF-lo of around 20 or less produces a deep stop. I will try to find the source I remember seeing it from.
ETA- found a couple
Gradient Factors in a Post-Deep Stops World

Decompression Theory Pt 4 - International Training - SDI | TDI | ERDI | PFI

That actually makes a lot of sense, surface tension will keep bubbles together and prevent the gas trapped inside them from participating in the offgasing I assume?

I think- someone correct me if it’s way off the mark- the “slower tissues” (in which the N2 half time is longer) off gas more slowly, due to the longer HT. So what happens is when you do stops that are too deep, those tissues just keep on gassing N2.
For those tissues, the pressure gradient is just not large enough for efficient off gassing.
I think that the decompression stops are not based on those slow tissues, rather than a medium- paced one (idk about that), so you’ll off-gas in the other tissues at a way faster rate than needed.
Which could result in DCS by, ironically, trying to be more careful. (maybe)

Thanks a lot for all this info, really appreciate your time. If you have any resources you can recommend (articles, posts or papers) please share

When I get home in an hour-30 ish I’ll find some!

 

[LFMarm]

I'm not sure I understand what you mean. Surely the NDL can be exceeded irrespective of both GF99 readings and GFHigh setting (simplest example I can think of is a slow but continuous descent where offgassing never even begins to occur)

What I mean is that if you dive a GF high lower than 99 (let’s say 85) you go into deco as soon as your SurfGF reaches 85, which is well below GF99 as OP was suggesting. How soon you go from NDL to deco for a given profile is a function of the conservatism chosen in the deco model

 

[LFMarm]

From what I remember, a GF-lo of around 20 or less produces a deep stop. I will try to find the source I remember seeing it from.
ETA- found a couple
Gradient Factors in a Post-Deep Stops World

Decompression Theory Pt 4 - International Training - SDI | TDI | ERDI | PFI

I think- someone correct me if it’s way off the mark- the “slower tissues” (in which the N2 half time is longer) off gas more slowly, due to the longer HT. So what happens is when you do stops that are too deep, those tissues just keep on gassing N2.
For those tissues, the pressure gradient is just not large enough for efficient off gassing.
I think that the decompression stops are not based on those slow tissues, rather than a medium- paced one (idk about that), so you’ll off-gas in the other tissues at a way faster rate than needed.
Which could result in DCS by, ironically, trying to be more careful. (maybe)

When I get home in an hour-30 ish I’ll find some!

Deco stops in the Bulhman model are based on the worst compartment, not necessarily the ones in the middle. If you look at simulations, you see that the critical compartment usually changes over time.

 

[L13]

Why do I keep seeing 40/70 as a commonly used profile for tech diving then? Are you saying modern thinking is 80/80 or something flat like that?

40/70 would be a common recommendation 5 or 10 years ago. As @tbone1004 said, the recommendation now is GF low about 80% of GF high, and in any case greater than 50%.

The first deeper stops are driven by the faster tissues that have absorbed the most nitrogen, but clear fast making the stops short. The slower tissues haven't absorbed as much nitrogen, so you can go shallower to off gas, but take longer to clear.

In the past, with lower GF low's and deeper first stops, the first stop was deep enough that the slower tissues were still on gassing during the stops requiring longer shallow stops and offsetting the advantage of reducing micro-bubbles.

 

[kaylee_ann]

Deco stops in the Bulhman model are based on the worst compartment, not necessarily the ones in the middle. If you look at simulations, you see that the critical compartment usually changes over time.

Ahh so the one that's closest to that supersaturation point, or the one reaching the GF-low value? sorry if that sounds dumb, I'm absolutely exhausted lol

 

[L13]

Ahh so the one that's closest to that supersaturation point, or the one reaching the GF-low value? sorry if that sounds dumb, I'm absolutely exhausted lol

You calculate the ceiling for each tissue (the depth at which it reaches the GF limit), The one with the lowest ceiling is the one that determines the next stop. At first it will be one of the fastest tissues, but each shallower stop will be limited by a slower tissue than the previous stop. When you get to the stop, the limiting tissue will be the one with the greatest supersaturation, but will drop below the next slower tissue before you go to the next stop.

At the first stop, the tissue with the highest supersaturation will also be the one closest to GF-low, but it will drop of quicker than the next tissue. At the start of each stop, the tissue with the highest supersaturation will be the limiting tissue and will be the one closest to the GF limit.

 

[kaylee_ann]

Thanks a lot for all this info, really appreciate your time. If you have any resources you can recommend (articles, posts or papers) please share

Here's a few of my favorites. They're relatively math-heavy, I will let you know. It isn't all math though, there's lots of general information too. These were posted in a comment on my deco calc thread, and of course, I forget their username for credit.

Also, not included here is the Deco for Divers book. Also, I've linked a presentation from Simon Mitchell. It helped clear up a lot of what I wasn't understanding when I first began studying deco theory.

 

[inquis]

so the one that's closest to that supersaturation point,

There is not a "supersaturation point". Tissues have the supersaturation values that they have. It's also not necessarily the tissue with highest tissue tension/pressure. (Faster tissues can handle greater tensions, according to Buhlmann.) There is a GF99 for each tissue compartment, and the controlling tissue is the one with highest GF99.

GF99 = fraction of the tissue pressure from ambient to the M-Value
GF99 = (P_tissue - P_ambient) / (M_value - P_ambient)
(P_xxx is the pressure of that whatever)

 

[inquis]

the tissue with the highest supersaturation will be the limiting tissue

I think there is a subtlety that's not coming across in your writing. Supersaturation is the amount over ambient (P_tissue - P_ambient). What is important is the supersaturation RELATIVE to how far the M-Value is over ambient. It's possible for a tissue with a lower supersaturation to control if it's M-Value is also low enough.